1. Field
The present disclosure relates to aerial image measurement, and more particularly relates to measuring an aerial image produced by an optical lithography system.
2. Description of the Related Art
FIG. 1 shows the configuration of a typical optical lithography system 1 used for the manufacturing of semiconductor devices. A wafer 3 is positioned on wafer stage 6, and illumination system 4 illuminates a pattern 5 on reticle 2 thereby generating light beams that are projected onto the wafer 3 by the projection lens 7 to create an image 8 corresponding to the pattern 5. Typical optical projection lithography systems 1 include an illumination system 4 that may use one or more illumination source points to generate beams that form the image 8 projected onto wafer 3.
To evaluate the effects of lens aberrations, illumination conditions, and other factors that affect the image performance of the lithography system 1, an aerial image corresponding to image 8 is measured. A measuring device 9 for measuring the aerial image is positioned on wafer stage 6, and by moving wafer stage 6 along the x and/or y direction, the aerial image can be measured by measuring device 9.
It should be understood that there is a difference between the aerial image and the measuring device's measurement of the aerial image. The aerial image is a projected image that cannot be viewed normally, but which would have been imaged onto wafer 3 if wafer 3 had been positioned by wafer stage 6 beneath projection lens 7. Instead, since aerial image measuring device 9 has been positioned beneath projection lens 7, the aerial image is not imaged but rather is measured. Measurement is ordinarily performed by sampling the aerial image through slit apertures that form part of measuring device 9, often coupled with movement of measuring device 9 so that different regions of the aerial image are sampled.
One goal of aerial image measurement is to calculate the actual aerial image that would have been projected and imaged onto wafer 3 if wafer 3 had been present, based on the measured aerial image. This calculation is often hampered by optical properties of measuring device 9, such that it is sometimes difficult to recreate an accurate depiction of the actual aerial image based on measurements obtained by measuring device 9.